1. Field of the Invention
The present invention relates to 1-(2-phenoxymethylphenyl)piperazine compounds having activity as serotonin (5-HT) and norepinephrine (NE) reuptake inhibitors. The invention also relates to pharmaceutical compositions comprising such compounds, processes and intermediates for preparing such compounds and methods of using such compounds to treat a pain disorder, such as neuropathic pain, and other ailments.
2. State of the Art
Pain is an unpleasant sensory and emotional experience associated with actual or potential tissue damage, or described in terms of such damage (International Association for the Study of Pain (IASP), Pain Terminology). Chronic pain persists beyond acute pain or beyond the expected time for an injury to heal (American Pain Society. “Pain Control in the Primary Care Setting.” 2006:15). Neuropathic pain is pain initiated or caused by a primary lesion or dysfunction in the nervous system. Peripheral neuropathic pain occurs when the lesion or dysfunction affects the peripheral nervous system and central neuropathic pain when the lesion or dysfunction affects the central nervous system (IASP).
Several types of therapeutic agents are currently used to treat neuropathic pain including, for example, tricyclic antidepressants (TCAs), serotonin and norepinephrine reuptake inhibitors (SNRIs), calcium channel ligands (e.g., gabapentin and pregabalin), topical lidocaine, and opioid agonists (e.g., morphine, oxycodone, methadone, levorphanol and tramadol). However, neuropathic pain can be very difficult to treat with no more than 40-60% of patients achieving, at best, partial relief of their pain (Dworkin et al. (2007) Pain 132:237-251). Moreover, all of the therapeutic agents currently used to treat neuropathic pain have various side effects (e.g., nausea, sedation, dizziness and somnolence) that can limit their effectiveness in some patients (Dworkin et al. supra).
SNRIs, such as duloxetine and venlafaxine, are often used as first line therapy for treating neuropathic pain. These agents inhibit the reuptake of both serotonin (5-hydroxytrypamine, 5-HT) and norepinephrine (NE) by binding to the serotonin and norepinephrine transporters (SERT and NET, respectively). However, both duloxetine and venlafaxine have higher affinity for SERT relative to NET (Vaishnavi et al. (2004) Biol. Psychiatry 55(3):320-322).
Preclinical studies suggest that inhibition of both SERT and NET may be necessary for maximally effective treatment of neuropathic and other chronic pain states (Jones et al. (2006) Neuropharmacology 51(7-8):1172-1180; Vickers et al. (2008) Bioorg. Med. Chem. Lett. 18:3230-3235; Fishbain et al. (2000) Pain Med. 1(4):310-316; and Mochizucki (2004) Human Psychopharmacology 19:S15-S19). However, in clinical studies, the inhibition of SERT has been reported to be related to nausea and other side effects (Greist et al. (2004) Clin. Ther. 26(9):1446-1455). Thus, therapeutic agents having more balanced SERT and NET affinity or slightly higher NET affinity are expected to be particularly useful for treating chronic pain while producing fewer side effects, such as nausea.
Thus, a need exists for novel compounds that are useful for treating chronic pain, such as neuropathic pain. In particular, a need exists for novel compounds that are useful for treating chronic pain and that have reduced side effects, such as nausea. A need also exists for novel dual-acting compounds that inhibit both SERT and NET with fairly high affinity (e.g., pKi≧7.0 or Ki≦100 nM) and balanced inhibition (e.g., a SERT/NET binding Ki ratio of 0.1 to 100).